"If you do not believe, it is because you are not trustworthy." Isaiah
7:9

Isaiah explains why people may have difficulty believing in Yahveh: they are not themselves trustworthy.
In other words, if I am reliable, and I know that people can trust me to keep my word to perform and deliver, then
I will have little difficulty in believing and trusting Yahveh to perform and deliver. Lack of trust in Yahveh is thus a reflection
of one's own lack of trustworthiness.

The Torah
tells us that those who find defects in others must themselves be defective in the same way. Isaiah is simply applying this
principle to trust and faith.